Engine "bogging" when trying to go WOT!!!

2000 Pontiac Grand Prix GTP- 3.25" Thrasher pulley
- Thrasher CAI
- Other Guys Ceramic Coated Headers
- "Test Pipe"
- U-bend removal
- 2.5" exhaust, Magnaflow mufflers
- NGK TR6
- Taylor Spiro 8mm Wires
- DHP PCM


About 3 months ago, I had to replace my flexplate since the stock one cracked. I took it to a tranny shop and had it replaced with a harden flexplate. during the process of being in the shop, the mechanic(s) didn't disconnect the battery, but only moved it out of the way. Once the installation was complete, they put the battery back in place, but grounded it on the car in the process. As a result, they shorted out my ignition module under the coil packs. The shop replaced the unit (free of charge of course), and then they sent me on my way.

Immediately out of the shop, I noticed that the car was not responding in the same way. When I would try to go WOT, the power would surge/"bog" at 4000 RPM, the same feeling when the stock fuel cut-out would activate when approaching redline. In park, it does the same thing. But, I have the DHP PCM which removes the stock rev limiter, and allows for the 4-2 and 3-1 shifting, which no longer occurs either since I get that damn surging. The car will accelerate more normally if I do maybe 50-60% throttle. The car seems underpowered in "D" when given gas, but performs better in 3-2-1.

Additionally, the car has a weird idle while not in motion, such as in drive at a stoplight. It will idle around 800RPM, and then kinda dip down to 500RPM, and the engine will sputter and then kick back up to 800, or sometimes the engine will just die. This has happened a few times when I brake and come to a stop at a red light. It also idles somewhat rich, but I still get 18-20MPG city and 25+ highway. I sometimes get a random misfire at highway speeds as well. I have had the car scanned using a Snap-On OBD-II diag comp, but it only showed the historical codes for the misfire.. no current codes!

I have checked the MAF to ensure that no "crud" has caked on it. Some other GP people have mentioned that the shorting of the ICM could have also damaged the crank sensor, and/or cam sensor. Anyone have any experiences with this problem or could offer their 2 cents??

This may sound stupid but where was the harden flexplate from? Was the shop sure it was for a GTP and not a GT? May sound dumb but if that was the only thing changed then maybe they bought a flexplate for a GT and not a GTP. I know there different but not sure how.

MixableLlama:
The flexplate was not the only thing replaced, and yes, I am almost certain that they installed a GTP flexplate. I would think it questionable if a tranny shop didn't know the difference. The other item replaced was the ICM that those fcukers shorted out when they grounded the battery on the car. Problems started after that...

I don't think a flexplate would cause the problems I am having though? Give me your reasons as to why you think it would so I can take them into consideration.

It was just a thought, I have been screwed by shops saying that they fixed a problem only for it to still be wrong. Then again where I live in, Flint, MI, people are always out to screw you here. It sucks. The flexplates looks similar though, thats all i ment. I know if you get them mixed up it not very good but I cant remember were on clubgp that I read what happens if you get them mixed up.

If it were me I would be at the shop bitchin till they fixed it. I dont know how much time you have but I would be callin everyday till they finaly would look at it and tell you whats wrong.

Already tried that route, but everytime they check it, they can't find anything wrong with it. This was more than 3 months ago, and after the first month of dealing with them, I just said fcuk it and went back to my private mechanic.

Lessons Learned: don't take performance cars to "hole-in-the-wall Jose's"

High probability the screw up damaged the PCM and other components such as the MAF, IAC, TPS, CKPS even though no DTC error codes are set. Problem will be sorting which component or components. Another thing that happens when tranny shops remove the transmission is they either forget the grounds or leave them loose. Check on all the grounds on the engine/tranny grounding stud. The ICM has a ground going to that stud and what occurs is when loose or disconnected it will not only blow the ICM but possibly damage the PCM sense 12v input. The input wires to the PCM will literally fry. I recall they were either red or orange in color coding. So I'd recommend inspecting all of that wiring leading up to the PCM to boot.

thanks BN

The flexplate was not the only thing replaced, and yes, I am almost certain that they installed a GTP flexplate. I would think it questionable if a tranny shop didn't know the difference.

I find it questionable that the tranny shop didn't disconnect the battery...

I don't think a flexplate would cause the problems I am having though? Give me your reasons as to why you think it would so I can take them into consideration.

sounds like your problem is def. electrical, not harwdware...the fact that they shorted out the ICM makes it more likely that they shorted something else. Bad grounds can cause seriously confussing diagnostic issues, so not to be a copycat, but i agree with bnaylor...

Where did you get the DHP from? If the short damaged the .bin file that could be the problem also. i think either ZZP or 3800perf. will reflash the PCM once for free...might be worth a shot

In the process of determining which component is bad and having down time without a PCM, personally, I'd check out the easy stuff first starting with the MAF sensor. The erratic idle and high rpm dropout is a symptom of a bad MAF sensor. The PCM will send a 5v reference signal to each sensor that could cause the problem. Same applies to the TPS and IAC. If the PCM is the cause a loss of reference voltage may be detected. All you need is a multimeter.

On TPS (throttle position sensor) , it provides a feedback of .4 volts and 4.0 volts back to the PCM. .4 volts at idle and 4.0 volts at WOT. It should be linear in voltage output from low to high rpms. On IAC (idle air controller) start by removing it and cleaning with carb cleaner. Check pintle for proper operation. Check reference voltage.

On MAF, 5 volt reference voltage can be checked but outputs back to the PCM are more difficult to check or verify. The MAF can be unplugged for troubleshooting purposes and the PCM should detect absense of MAF signal causing it to use the VE table pre-programmed values. Take it for a test drive with MAF unplugged.

If the PCM was damaged I would believe that it would be generating some DTC error codes indicating loss of serial data or internal error codes but I would not rule it out.